Can we see defects in capacitance measurements of thin‐film solar cells?

Abstract

Thermal admittance spectroscopy and capacitance‐voltage measurements are well established techniques to study recombination‐active deep defect levels and determine the shallow dopant concentration in photovoltaic absorbers. Applied to thin‐film solar cells or any device stack consisting of multiple layers, interpretation of these capacitance‐based techniques is ambiguous at best. We demonstrate how to assess electrical measurements of thin‐film devices and develop a range of criteria that allow to estimate whether deep defects could consistently explain a given capacitance measurement. We show that a broad parameter space, achieved by exploiting bias voltage, time, and illumination as additional experimental parameters in admittance spectroscopy, helps to distinguish between deep defects and capacitive contributions from transport barriers or additional layers in the device stack. On the example of Cu(In,Ga)Se2 thin‐film solar cells, we show that slow trap states are indeed present but cannot be resolved in typical admittance spectra. We explain the common N1 signature by the presence of a capacitive barrier layer and show that the shallow net dopant concentration is not distributed uniformly within the depth of the absorber.

Emerging Chalcogenide Thin Films for Solar Energy Harvesting Devices

“…The commonly used circuit model in capacitance measurements contains a capacitor in parallel to a resistor. Although this model is suitable for ideal p–n junctions, nonidealities such as temperature-dependent series resistance, transport barriers, etc. can lead to errors.…”

Efficient Sb2(S,Se)3 solar cells via monitorable chemical bath deposition

“…7b and c, we obtained the inflection frequencies ( f 0 ) for each of the defect levels as a function of temperature by plotting − f d C /d f ∼ f curves. Thus, the activation energy ( E a ) of the defect levels were calculated according to eqn (3), 41,44,45 where f is the temperature-dependent inflection frequency for a defect response, T is the temperature in Kelvin, and ν 0 is the attempt-to-escape frequency. As exhibited in Fig.…”

“…7b and c, we obtained the inection frequencies (f 0 ) for each of the defect levels as a function of temperature by plotting Àf dC/df $ f curves. Thus, the activation energy (E a ) of the defect levels were calculated according to eqn (3), 41,44,45…”

Absorber composition: A critical parameter for the effectiveness of heat treatments in chalcopyrite solar cells

“…For calculations, we have used a value of ɛ equal to 10, as is often used in literature 31,32 . This SCR width does contain contributions from the buffer layer 33,34 . We therefore discuss only changes and not the absolute values.…”